Earlier patents, that do not appear to anticipate the present invention, and therefore, are disclosed as background information, show that it is known to use conventional hot-dip galvanizing processes to apply conventional hot-dip zinc coatings to the surface of steels for corrosion protection. Such galvanizing processes generally involve heating a steel substrate under controlled conditions, immersing the steel into a molten bath of a coating metal such as zinc or a zinc alloy, and cooling the coated material for further downstream processing and/or subsequent use.
In one exemplary downstream process, known in the coating art as galvannealing, the conventional hot-dip zinc coated material is heated in an annealing furnace, and the reheated zinc coating reacts with the steel material at the interface of the substrate forming a zinc alloy coating on the base steel material during the annealing process. Such galvannealed material is advantageous in that the coated surface exhibits good paint-adherence properties.
One well-known problem related to the use of conventional hot-dip zinc-coating processes is that it is difficult to apply a good quality hot-dip zinc coating to high strength dual phase steels. In order to manufacture high strength steels, it is necessary to add strengthening alloys during the steelmaking process. In the present invention, where strengthening is achieved through the formation of a dual phase microstructure (ferrite plus, primarily martensite), it is necessary to make alloy additions with elements such as Mn, Si, Mo, and Cr. Many of these alloying elements can have a detrimental effect on the coating quality due to zinc dewetting when coated by hot-dip galvanizing. Elements such as Mn, Si, and Cr, that are easily oxidized, and therefore, are troublesome when they are present above levels normally found in the above-mentioned low or ultra low carbon steels. For example, in a galvanneal product, when manganese and silicon are added to high strength steels, and when such high strength steels are hot-dip zinc coated in a conventional continuous hot-dip coating line, the high atmosphere temperature in the recrystallizing/annealing furnace reduces the iron and oxidizes silicon and manganese. This forms either separate or complex manganese and silicon oxides that prevent good zinc adhesion to the steel substrate resulting in bare (uncoated) spots on the coated steel surface. It is expected that other alloy additions, that form more stable oxides than iron, will also produce similar hot-dip zinc coating difficulties during galvanizing process.
The problem of coating high strength steels, particularly those containing large amounts of manganese is recognized in EP 1 041 167 to Kawasaki Steel. This publication admits that it is very difficult to manufacture high strength steel on a hot-dip galvanizing line due to the presence of alloying elements added for strength, and specifically notes the problems with the presence of manganese oxides and the difficulties in zinc coating when these oxides are present.
The Kawasaki Steel EP publication attempts to eliminate the dewetting problems encountered when coating high strength steels with zinc through the use of a specific alloy and a complicated heating cycle. More particularly, Kawasaki Steel employs a particular composition in a steel sheet form and heats the composition to a prescribed level to cause dispersion of a band structure comprising a secondary phase, mainly cementite, pearlite, and bainite, and only partially martensite and residual austenite, to a prescribed extent.
Kawasaki recognizes the problems when the manganese content is high for a steel that is to be galvanized, and suggests that the steel should be first annealed on a continuous annealing line and then heated as part of the galvanizing process. Kawasaki does suggest that a single high temperature heating prior to galvanizing can be done (but provides no specifics as to such a process), and acknowledges that this type of high temperature heating deteriorates the steel surface. To avoid this problem, Kawasaki suggests a two step heating process including first heating the steel in a continuous annealing line at a temperature of at least 750° C., cooling the steel, pickling the steel surface, and then heating the steel between 650° and 950° C. just prior to dipping the steel in the galvanizing hot-dip bath. As part of the second heating step, Kawasaki suggests that the dew point temperature be controlled within −50° C. and 0° C. The steel exemplified in Kawasaki utilized 2.0% by weight manganese, 0.15% by weight molybdenum, and about 0.09% carbon, and the example used a heating-pickling-heating-galvanizing process to coat the material, requiring the use of a continuous anneal line and a galvanizing line.
While Kawasaki suggests ways to avoid the problems of coating high strength steels, the proposed solutions are still disadvantageous in that a special two step processing is required. Thus, when attempting to coat these types of steels, modifications must be made to the conventional galvanizing line, or extra processing steps are required.
Another solution proposed by earlier references related to hot-dip coating problems in high strength steels is electrolytically plating the steel substrate with nickel or an iron-boron alloy as described in U.S. Pat. No. 4,913,785, assigned to Nisshin Steel. Japanese Publication No. JP A 60 262950 also teaches electroplating nickel on steel substrates containing silicon and aluminum as a precursor step for galvanizing.
It has also been suggested that the hydrogen content in the annealing furnace be increased to prevent zinc dewetting on manganese-containing high strength interstitial free steel, see “Hot-dip Galvannealing of Interstitial Free Steel Strengthened by Manganese,” Zhang et al., Galvatech '95 Conf. Proc., pp. 115-120. It has also been reported that the dew point of the annealing atmosphere should be increased to improve zinc dewetting on a high strength Mn-containing Ti—Nb interstitial free steel substrate, see “Influence of the Dew Point of the N2-H2 Atmosphere during Recrystallization Annealing on the Steel Surface of TiNb IF High Strength Steels”, Hertveldt et al., 41st MWSP Conf. Proc., ISS, Vol. XXXVIII, 1999, pp. 227-234. In this article, it is suggested that increasing the dew point allows the manganese oxides to form internally in the steel rather than on the surface.
In view of the added burdens imposed by the earlier solutions for overcoming problems associated with applying hot-dip zinc coatings to high strength steels, and in particular, to the solutions related to the use of zinc dewetting techniques, there still exists a long felt need for a more simple, and more effective, method to manufacture a hot-dip zinc coated high strength steel product that exhibits good coating adhesion properties. The present invention responds to this need with the discovery that conventional hot-dip zinc coating processes may be used to galvanize or galvanneal a manganese-molybdenum-carbon-containing high strength dual phase steel composition to manufacture a conventional hot-dip zinc coated product having good coating adhesion properties.